1. Field of the Invention
The present invention relates generally to a network system including a Circuit Switched (CS) domain, an Internet Protocol (IP) Multimedia Subsystem (IMS) domain and a Packet Switched (PS) domain, and in particular, to a method and system in which a network sends an IMS reregistration request message to a User Equipment (UE) upon occurrence of a Proxy Call Session Control Function (P-CSCF) failure.
2. Description of the Related Art
A CS scheme, the typical scheme for voice services and real-time services, means a scheme in which a CS-based fixed call route is established between one user and its counterpart user. IMS is suitable for data services, messaging services and file transfer services, as it improves transmission efficiency and guarantees transmission stability with the use of a PS-based unfixed call route based on Internet Protocol (IP). Such an IMS can support not only the simple point-to-point call but also the point-to-multipoint call in which multiple users simultaneously take part. In the CS scheme, transmission of messages and user traffics is achieved over a CS call, and in IMS, the transmission of messages and user traffics is achieved over an IMS session.
FIG. 1 illustrates a configuration of a network to which the present invention is applicable.
Referring to FIG. 1, the network, to which the present invention is applicable, is roughly divided into a radio access network 106 and a core network 102. A User Equipment (UE) 104 accesses the core network 102 over the radio access network 106. The core network 102 is roughly divided into a CS domain 112, a PS domain 110, and an IMS domain 108.
The CS domain 112 is composed of entities supporting a CS call. Among the entities, a Mobile Switching Center (MSC; not shown) processes the call signals incoming and outgoing from/to a mobile base station, and performs a centralized control function so that the mobile base station can efficiently operate. A Media Gateway (MGW; not shown) is a gateway supporting the bearer traffics and control traffics. A Short Message Service (SMS) Service Center (SMS-SC) 124 serves to transfer SMS, or SMS messages, to users. A Visitor Location Register (VLR) 122 manages all the user data necessary for user call processing and mobility management.
The PS domain 110 supports PS-based services, and is roughly divided into a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN).
A Home Subscriber Server (HSS) 120 or a Home Location Register (HLR), which is an entity used for both the CS domain 112 and the PS domain 110, manages subscription information, service information, and location information of the users. The HSS 120 is an evolved form of the HLR, and either of the two entities can be used in the present invention.
The IMS domain 108 is composed of entities supporting the IMS session. Among the entities, a Proxy-Call Session Control Function (P-CSCF) 116, a Serving-Call Session Control Function (S-CSCF) 114, and an Application Server (AS) 118 are related to the present invention. The P-CSCF 116 is an entry point for the IMS domain 108 of the UE 104, and with the use thereof, the UE 104, when accessing the radio access network 106, receives a P-CSCF address from the radio access network 106 or acquires it through a Dynamic Host Configuration Protocol (DHCP) query. The S-CSCF 114, an entity for performing the most important function of establishing an IMS session, serves to receive and process a Session Initiation Protocol (SIP) message delivered from the UE 104. That is, to provide the service as requested by the UE 104, the S-CSCF 114 delivers SIP messages to the entities in the IMS domain 108 so that the session is completed. The AS 118 is an entity for providing applications to the users, like an E-mail server.
In the network with the foregoing configuration, the UE should first perform IMS registration in order to receive IMS services. Through this process, a P-CSCF and an S-CSCF that serve the UE are determined. However, when it is impossible to serve the UE as failure occurs in the P-CSCF, the UE cannot receive the IMS service even though the radio bearer is alive between the UE and the radio access network. For example, if an incoming call for the UE arrives when P-CSCF failure has occurred, the network cannot deliver the call to the UE.
Conventionally, upon occurrence of the P-CSCF failure, the UE perceives the need for IMS reregistration and performs it as follows. The UE performs IMS registration by transmitting ‘REGISTER’, which is one of the SIP Methods, to an IMS network. Thereafter, the UE refreshes the IMS registration by sending REGISTER to the IMS network at stated intervals. After sending REGISTER, if a response message thereto is not received, the UE can perceive the occurrence of the P-CSCF failure. However, when the time interval for which the REGISTER is sent is long and the P-CSCF failure has occurred immediately after the refreshment of the IMS registration, the user cannot receive the call being delivered thereto for quite a long time.